PLESIOSAURUS, an extinct marine reptile belonging to the Order Sauropterygia, which characterized the Mesozoic period and had an almost world-wide distribution (see 'REPTILES). The animal is best known by nearly complete skeletons from the Lias of England and Germany. It was named Plesiosaurus (Gr. more-lizard) by W. D. Conybeare in 1821, to indicate that it was much more nearly a normal reptile than the strange
(From a memoir by Professor W. Dames in the Abhandlungen der kg. preuss. Akad. d. Wiss.)
Plesiosaurus guilelmi-imperatoris, restored.
Ichthyosaurus, which had been found in the same Liassic forma-
tion a few years previously. It has a small head, a long and
slender neck, a round body, a very short tail, and two pairs
of large, elongated paddles. The snout is short, but the gape
of the mouth is wide, and the jaws are provided with a series
of conical teeth in sockets, much like those of the living gavial
i Magdal6nien from the caves of Madelaine, Perigord.
2 Salutr6, Bourgogne.
3 Chelles, near Paris. Other subordinate stages are the Moustbrien from Moustier, Dordogne, and Acheul6en, Saint Acheul.
of Indian rivers. The neck, though long and slender, must have been rather stiff, because the bodies of the vertebrae are nearly flat-ended, while they bear short ribs: it could not have been bent in the swan-fashion represented in many restorations. The other vertebrae are similarly almost flat-ended and firmly united, but there is no sacrum. The ribs are single-headed, and in the middle of the trunk, between the supports of the paired limbs, they meet a dense plastron of abdominal ribs. The short tail is straight and rapidly tapering, but one specimen in Berlin suggests that it was provided with a rhomboidal flap of skin in a vertical plane. The bones in the ventral wall of the body which support the paired limbs are remarkably expanded, and those of the pectoral arch have often been compared with the corresponding bones of turtles. The limbs are elongated paddles, with five complete digits, of which the constituent bones (phalanges) are unusually numerous. The only traces of skin hitherto discovered suggest that it was smooth. The reptile must have been almost exclusively aquatic, feeding on cuttlefishes, fishes and other animal prey. It propelled itself chiefly by the paddles, scarcely by the tail.
The typical species is Plesiosaurus dolichodeirus, from the Lower Lias of Lyme Regis, which attains a length of about three metres. Other species from the same formation seem to have measured five to six metres in length, and there are species of allied genera from the Upper Lias which are probably still larger. A fine large skeleton from the Upper Lias of Wurttemberg, now in the Berlin Museum, is named Plesiosaurus guilelmiimperatoris (see figure above). Cryptoclidus, known by complete skeletons from the Oxford Clay of Peterborough, differs very little from Plesiosaurus. The Cretaceous Cimoliosaurus, found in North and South America, Europe and New Zealand, is also very similar. The fossilized contents of the stomach in some of the later Plesiosaurs show that these reptiles swallowed stones for digestive purposes like the existing crocodiles.
(Monogr. Palaeont. Soc., 1865) ; W. Dames, paper in Abhandl. k. preuss. Akad. Wiss. (1895), p. I. (A. S. Wo.)